Generally, in a manual transmission (manual gear-shift transmission), when the driver operates the change-lever manually, the force applied by the driver in the operation is transmitted through a gear-shifting device to a shift fork, so that the shift fork sets a corresponding clutch (for example, a synchromesh mechanism) into engagement, establishing a desired speed change ratio for the transmission. Such clutches are arranged over either of the two parallel shafts of the transmission, in correspondence to permanently meshing gear trains of various gear ratios, which gear trains are disposed over the two parallel shafts in parallel with one another.
Such a manual transmission comprises a system (gear-shifting device) that transmits the operational force (shifting force) applied by the driver at the change-lever. For example, Japanese Laid-Open Patent Publication No. 2003-14114 discloses such a system that comprises a shift selector shaft, a shift arm and a plurality of shift pieces. In this case, the shift selector shaft, which is equipped with the shift arm, is movable in its axial direction and rotatable axially in correspondence to the operation of the change-lever, which is connected through connecting members like a shift cable. Shift forks are provided in the same number as the gear trains that are used as the speed change ratios of the transmission, and each shift fork has, as a one-piece body, a shift fork shaft, on which a corresponding shift fork is provided. When the driver operates the change-lever appropriately, the shift arm is shifted in the axial direction of the shift selector shaft and engages selectively with one of the shift pieces, and then the shift arm is rotated around the shift selector shaft to shift the corresponding shift fork shaft in its longitudinal axial direction. As a result, the operational force applied at the change-lever is transmitted selectively to the corresponding one of the shift forks for a gear shift.
Into such a clutch mechanism, a synchromesh mechanism is adopted to achieve a smooth clutching operation for a gear shift, which is controlled by the operation of the change-lever. In this case, it is desirable to lighten the burden of the driver in operating the change-lever, so there are a plurality of methods of reducing the force required for operating the change-lever and for transmitting this reduced operational force securely to the shift fork to achieve a gear shift. For example, there is a method for increasing the leverage or stroke of the change-lever. Another method tries to increase the capacity of the synchromesh mechanism to reduce the load generated in the clutch during the synchronization.
However, there is a limit to the increase of the leverage that is effected by increasing the stroke of the change-lever because the installation space of the change-lever in the interior of a vehicle is limited. Also, it is not desirable from the point of view of the driver's maneuverability of the change-lever. Moreover, if the leverage is increased, then the stroke of the shift fork must be reduced in inverse proportion. This may result in an impairment to the secure clutching operation. On the other hand, the increasing of the synchronization capacity results in an increase in the number of the parts constituting the transmission and can lead to an increase in the manufacturing cost of the transmission.